1. Field of the Invention
The present invention concerns a method for the radioelectric synchronization of slave stations by a master station. This method is particularly usable to the synchronization of azimuth and elevation (or bearing) stations of a Microwave Landing System (MLS). The invention also concerns devices for reducing to practice this method.
2. Description of the Prior Art
It will be recalled that a MLS supplies an aircraft with different data called "functions" as to its position, especially its angle of azimuth and its angle of elevation in relation to the landing runway reference system. The MLS may further supply other associated functions such as the rear azimuth, for example, and a certain number of data, some of which are called "basic data" and others of which are called "auxiliary data". These data are alternately transmitted by the MLS from the ground in time multiplexing upon a single frequency, close to 5 GHz, according to the specifications standardized by the International Civil Aviation Organization (OACI), annex 10, paragraph 3-eleven.
Each of the data transmitted, especially as to the azimuth and to the elevation, is divided into two successively transmitted parts:
a preamble, the main role of which is to supply the aircraft with identification of the transmission that will immediately follow. This preamble is transmitted by a sectorial antenna, i.e. an antenna having a stationary radiation diagram covering the whole of the zone, or sector, covered by the MLS. The preamble is a binary word transmitted in Differential Phase Shift Keying (DPSK) according the OACI standards; PA1 angular data per se, transmitted by means of an electronic scanning antenna, according to the well-known Time Reference Scanning Beam (TRSB) procedure. PA1 a receiving antenna, operating at about 5 GHz according to OACI standards, directed towards the azimuth station so as to pick-up the scanning beam of this station when it passes; PA1 receiving means, operating at about 5 GHz, detecting the pulses sent by the scanning beam from the azimuth station; PA1 logic means for monitoring the transmission sequences; PA1 a clock of high precision and stability for ensuring the operating autonomy of the elevation station, even in the presence of temporary shut-down of receiving the signal transmitted by the azimuth station; PA1 logic means for performing the transmission of the sequences appropriate to the elevation station in the event of temporary loss of synchronization data.
FIG. 1 represents a usual disposition of the azimuth, elevation and rear azimuth stations near a landing runway.
The azimuth and elevation stations are generally positioned with respect to one another at a distance of several kilometers apart. The azimuth station (Az) is close to the end of the runway, marked P and having axis ZZ, and it transmits in the direction of the runway P thus allowing the aircraft (Av) to dispose of the angular azimuth data during the entire landing operation, even during the rolling onto the runway phase. The elevation station (S) transmits in the same direction but it is on the contrary close to the runway threshold, allowing the aircraft (Av) to be guide-controlled onto a path at a constant angle of elevation during landing and to be brought by this path upon entry of the runway. The rear azimuth station (A.sub.AR) is positioned at the runway entry, so as to be symmetrical to the azimuth station Az, and also transmits in the direction of the runway. Time-sharing operation upon a single frequency thus necessitates the existence of a time synchronizing link between the stations, in order to ensure the non-overlapping of the azimuth, elevation and rear azimuth transmissions.
FIG. 2 represents an embodiment of a complete OACI time sequence transmitted by a MLS.
The sequence is formed of a succession of identical "cycles" having a maximum duration of 615 ms. Each cycle itself is divided into a succession of 37 sequences" and of auxiliary data words.
FIGS. 3a and 3b, represent respectively two particular examples of what is called "sequence", of types bearing references 1 and 2.
In these two types, a sequence comprises a succession of time durations reserved respectively for the azimuth, for the elevation, for the rear azimuth and for the basic data.
The purpose of the synchronization is to ensure that the different functions, azimuth, elevation, rear azimuth, are transmitted within the time duration allocated thereto. Generally, the azimuth station acts as master station by generating the MLS cycle. In order to do this, the master station must send synchronization signals towards the other stations in order to allow them to transmit when required.
This synchronization can be carried out through a physical link, electric cable or optical fiber, for example. But it thus presents the drawback of being costly due to the civil engineering works that it requires, especially for the construction of trenches over distances which may reach several kilometers. This is the reason why it appears more worthwhile to carry out this synchronization through radioelectric means.
One solution to carry out this radioelectric synchronization uses go and return scanning of the scanning beam of the azimuth station. The principles of the scanning beam are already mentioned herein-above and will be described in further detail herein-below, with reference to FIGS. 3 and 4 which illustrates the principles of the respective transmissions in azimuth and in elevation.
From the azimuth station, according to what is set out herein-above, are transmitted two different radiations by two distinct antennae, which, for enhanced simplicity, have been represented on the same point A.sub.Z in FIG. 4. From point A.sub.Z there is represented the diagram of the preamble transmission, reference P.sub.AZ, transmitted by the sectorial antenna within the entire covering zone of the MLS system, which is represented in the drawing by an angle .alpha.. From this point A.sub.Z, there is further represented the diagram of a flat and vertical beam B.sub.AZ, called scanning beam, transmitted by an electronic scanning antenna. The beam B.sub.AZ performs at constant speed a first scanning then, after a dead time, a return scanning, and this occurs within a scanning zone forming an angle .beta..sub.Z on FIG. 4, which can be equal to or smaller than the previous covering angle .alpha.. In FIG. 4, .beta..sub.Z has been represented smaller than .alpha.; and an arrow R.sub.Z represents respectively the outward and inward scanning paths of the beam B.sub.AZ within scanning zone .beta. .sub.Z. An aircraft A.sub.V has been represented by way of example in an incorrectly aligned position relative to the axis ZZ of the runway.
According to the OACI standards, the angle .beta..sub.Z is comprised between a minimum of 20.degree., being divided with respect to the axis of runway ZZ into the semi-angles -.theta..sub.M =+.theta..sub.M =10.degree., and a maximum of 120.degree. maximum with -.theta..sub.M =+.theta..sub.M =60.degree.. The beam B.sub.AZ has an aperture of 1 to 4% in the plane of the figure and of about 15.degree. in the vertical plane.
FIG. 5 represents, in a manner analogous to that represented in FIG. 4, the principle of the elevation transmission.
FIG. 5 therefore shows the elevation station S from which are transmitted two beams by two distinct antennae. The sectorial antenna transmits the elevation preamble P.sub.S of which the diagram has been represented in the drawing. The other transmits a flat scanning beam B.sub.S, scanning the scanning zone of angle marked .beta..sub.S. This scanning is performed in the same way as for the azimuth scanning beam B.sub.AZ.
In this first type of solution, the go and return scanning of the scanning beam of the azimuth station, that compulsorily passes onto the elevation station, is used for the synchronization of this latter station. This implies that the elevation station be equipped, furthermore, with:
A first type of drawback inherent in this solution arises from the rounded shape and the considerable width of the pulses obtained by the passages of the scanning beam B.sub.AZ upon the elevation station (50 to 200 .mu.s), the too-slow fronts of which do not allow to obtain good precision upon the arrival time of the pulse, a precision which is desired to be in the order of a few .mu.s.
A second type of drawback arises from the need for very complex logic means in order to monitor the transmission sequences, taking into account the fact that the azimuth and elevation sequences follow one another in an irregular way in a MLS cycle, thereby compelling the slave stations (elevation station) to distinguish them from one another.
A second solution to the problem of the radioelectric synchronization of the elevation station by the azimuth station consists in using the preamble transmitted by the azimuth station. In this case, the elevation station must comprise the same supplementary means as in the first type of solution, with the exception that the receiving means must now decode the DPSK modulated transmissions.
Here again, this type of solution presents drawbacks, including the necessity of having a very sensitive MLS receiver, due to the fact that the transmission of the preamble by the sectorial antenna is carried out at a lower level than the transmission by the scanning antenna, and the complexity of the receiver that must be able to decode the DPSK modulation.
An object of the present invention is to overcome these various drawbacks. According to the invention, there is provided a method for the radioelectric synchronization of at least one slave station by a master station in a data communication system, comprising a plurality of successive steps of data transmission by said stations, said steps constituting a cycle, the method comprising a supplementary radioelectric transmission step by the said master station of a synchronization data coded in the form of a plurality of successive pulses, said synchronization data being periodically transmitted, at least once per cycle.